1. Field of the Invention
The present invention relates to the field of mass spectrometry. More particularly, the present invention relates to a mass spectrometer system and method that provides for improved high mass resolving power (MRP) and sensitivity via deconvolution of the spatial and temporal characteristics collected at the exit aperture of a quadrupole instrument.
2. Discussion of the Related Art
Quadrupoles are conventionally described as low resolution instruments. The theory and operation of conventional quadrupole mass spectrometers is described in numerous text books (e.g., Dawson P. H. (1976), Quadrupole Mass Spectrometry and Its Applications, Elsevier, Amsterdam), and in numerous Patents, such as, U.S. Pat. No. 2,939,952, entitled “Apparatus For Separating Charged Particles Of Different Specific Charges,” to Paul et al, filed Dec. 21, 1954, issued Jun. 7, 1960.
As a mass filter, such instruments operate by setting stability limits via applied RF and DC potentials that are capable of being ramped as a function of time such that ions with a specific range of mass-to-charge ratios have stable trajectories throughout the device. In particular, by applying fixed and/or ramped AC and DC voltages to configured cylindrical but more often hyperbolic electrode rod pairs in a manner known to those skilled in the art, desired electrical fields are set-up to stabilize the motion of predetermined ions in the x and y dimensions. As a result, the applied electrical field in the x-axis stabilizes the trajectory of heavier ions, whereas the lighter ions have unstable trajectories. By contrast, the electrical field in the y-axis stabilizes the trajectories of lighter ions, whereas the heavier ions have unstable trajectories. The range of masses that have stable trajectories in the quadrupole and thus arrive at a detector placed at the exit cross section of the quadrupole rod set is defined by the mass stability limits.
Typically, quadrupole mass spectrometry systems employ a single detector to record the arrival of ions at the exit cross section of the quadrupole rod set as a function of time. By varying the mass stability limits monotonically in time, the mass-to-charge ratio of an ion can be (approximately) determined from its arrival time at the detector. In a conventional quadrupole mass spectrometer, the uncertainty in estimating of the mass-to-charge ratio from its arrival time corresponds to the width between the mass stability limits. This uncertainty can be reduced by narrowing the mass stability limits, i.e. operating the quadrupole as a narrow-band filter. In this mode, the mass resolving power of the quadrupole is enhanced as ions outside the narrow band of “stable” masses crash into the rods rather than passing through to the detector. However, the improved mass resolving power comes at the expense of sensitivity. In particular, when the stability limits are narrow, even “stable” masses are only marginally stable, and thus, only a relatively small fraction of these reach the detector.
Background information on a system and method that utilizes a mathematical deconvolution process to analyze spatial characteristics provided by an arrayed source, is described and claimed in, U.S. Pat. No. 7,339,521, entitled, “ANALYTICAL INSTRUMENTS USING A PSEUDORANDOM ARRAY OF SOURCES, SUCH AS A MICRO-MACHINED MASS SPECTROMETER OR MONOCHROMATOR,” issued Mar. 4, 2008, to Scheidemann et al., including the following, “Novel methods and structures are disclosed herein which employ pseudorandom sequences to spatially arrange multiple sources in a pseudorandom source array. The pseudorandom source array can replace the single source in analytical instruments relying on spatial separation of the sample or the probe particles/waves emitted by the sources. The large number of sources in this pseudorandom source array enhances the signal on a position sensitive detector. A mathematical deconvolution process retrieves a spectrum with improved signal-to-noise ratio from the detector signal.”
Background information for a mass spectrometer system that provides for spatial detection of ions via a photo-emissive device, is described and claimed in, U.S. Pat. No. 4,810,882, entitled, “MASS SPECTROMETER FOR POSITIVE AND NEGATIVE IONS,” issued Mar. 7, 1989, to Bateman et al., including the following, “[t]he invention provides a mass spectrometer capable of detecting both positive and negative ions. Positive ions emerging from the mass analyzer strike a conversion electrode to release secondary electrons which pass through an annular electrode to strike a phosphor, releasing photons. Negative ions strike the surface of the annular electrode to release secondary electrons which also strike the phosphor, releasing photons. The photons are detected with a conventional photomultiplier. The electrodes are biased and disposed so that both positive ions and negative ions may be detected without changing the potentials applied to them.”
Background information for a system that uses an arrayed detector for ion collection is described in, “From the Infrared to X-ray: Advanced Detectors Set to Revolutionize Spectroscopy,” presented Mar. 8, 2009 at Pittcon by Bonner Denton, including the following, “[w]hole new generations of highly promising ion and electron detectors are being implemented by adapting and modifying a combination of technologies originally developed for visible CCD's and infrared multiplexer arrays. This new generation of ion and electron detectors is being implemented in configurations ranging from a single element suitable for quadrupole and time-of-flight ion mobility instruments to linear arrays for ion cycloidal and sector-based mass spectrometers. The latest results using these new techniques to read micro Faraday cups and arrays of finger electrodes will be presented. Since this approach is a high-sensitivity Faraday type coulombic detector, it is suitable for implementing high-density arrays in isotope ratio spectrometers and conventional mass spectrometers, as well as ultra high-sensitivity detectors for ion mobility spectrometers.” While the described detectors in the presentation provide information about the exit positions of ions, the described research does not make use of this information. Rather, the array is used to improve the total number of ions captured and is functionally equivalent to a single detector with enhanced sensitivity.
FIG. 1A shows example data from a conventional Triple Stage Quadrupole (TSQ) mass analyzer to illustrate mass resolving power capabilities presently available in a quadrupole device. As shown in FIG. 1A, the mass resolving power that results from the example detected m/z 508.208 ion is about 44, 170, which is similar to what is typically achieved in “high resolution” platforms, such as, Fourier Transform Mass Spectrometry (FTMS). To obtain such a mass resolving power, the instrument is scanned slowly and operated within the boundaries of a predetermined mass stability region. Although the mass resolving power (i.e., the intrinsic mass resolving power) shown by the data is relatively high, the sensitivity, while not shown, is very poor for the instrument.
FIG. 1B (see inset) shows Q3 intensities of example m/z 182, 508, and 997 ions from a TSQ quadrupole operated with a narrow stability transmission window (data denoted as A) and with a wider stability transmission window (data denoted as A′). The data in FIG. 1B is utilized to show that the sensitivity for a mass selectivity quadrupole can be increased significantly by opening the transmission stability window. However, while not explicitly shown in the figure, the intrinsic mass resolving power for a quadrupole instrument operated in such a wide-band mode often is undesirable.
The key point to be taken by FIGS. 1A and 1B is that conventionally, operation of a quadrupole mass filter provides for either relatively high mass resolving power or high sensitivity at the expense of mass resolving power but not for both simultaneously and in all cases, the scan rate is relatively slow. The present invention, however, provides for a system and method of operation that simultaneously provides for both a high mass resolving power and an increased sensitivity at higher scan rates, which exceeds current capabilities of quadrupole mass analyzers.
Accordingly, there is a need in the field of mass spectrometry to improve the mass resolving power of such systems without the loss in signal-to-noise ratio (i.e., sensitivity). The present invention addresses this need, as disclosed herein, by measuring the ion current as a function of both time and spatial displacement in the beam cross-section and then deconvolving the contributions of the signals from the individual ion species.